Intensity Modulated Radiation Therapy Fields Research Articles

SU-F-T-616: Comparison of Different Techniques Using RTOG0631 Guidelines in Spine SBRT

Purpose: Stereotactic body radiosurgery (SBRT) for spine metastases involves irradiation using a single high dose fraction. The purpose of this study was to investigate a Hybrid VMAT/IMRT technique which combines volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT) for spine SBRT in terms of its dosimetric quality and treatment efficiency using Radiation Therapy Oncology Group (RTOG) 0631 guidelines. Methods: 7 fields IMRT, 2 full arcs VMAT and Hybrid VMAT/IMRT were created for ten previously treated patients. The Hybrid VMAT/IMRT technique consisted of 1 full VMAT arc and 5 IMRT fields. Hybrid VMAT/IMRT plans were compared with IMRT and VMAT plans in terms of the dose distribution, spinal cord sparing, homogeneity, conformity and gradient indexies, monitor unit (MU) and beam on time (BOT). RTOG 0631 recommendations were applied for treatment planning. All plans were normalized and prescribed to deliver 18.0 Gy in a single fraction to 90% of the target volume. Results: The Hybrid VMAT/IMRT technique significantly improved target dose homogeneity and conformity compared with IMRT and VMAT techniques. Providing sharp dose gradient Hybrid VMAT/IMRT plans spare the spinal cord and healthy tissue more effectively. Although, both MU and BOT slightly increased in Hybrid VMAT/IMRT plans there is no statistically meaningful difference between VMAT and Hybrid VMAT/IMRT plans. Conclusion: In IMRT, a smaller volume of healthy tissue can be irradiated in the low dose region, VMAT plans provide better target volume coverage, favorable dose gradient, conformity and better OAR sparing and also they require a much smaller number of MUs and thus a shorter treatment time than IMRT plans. Hybrid plan offers a sinergy through combination of these two techniques with slightly increased number of MU and thus more treatment time.

SU-F-T-349: Dosimetric Comparison of Three Different Simultaneous Integrated Boost Irradiation Techniques for Multiple Brain Metastases: Intensity-Modulatedradiotherapy, Hybrid Intensity-Modulated Radiotherapy and Volumetric Modulated Arc Therapy

Purpose: To study the dosimetric impact of intensity-modulated radiotherapy (IMRT), hybrid intensity-modulated radiotherapy (h-IMRT) and volumetric modulated arc therapy(VMAT) for whole-brain radiotherapy (WBRT) with simultaneous integrated boost in patients with multiple brain metastases. Methods: Ten patients with multiple brain metastases were included in this analysis. The prescribed dose was 45 Gy to the whole brain (PTVWBRT) and 55 Gy to individual brain metastases (PTVboost) delivered simultaneously in 25 fractions. Three treatment techniques were designed: the 7 equal spaced fields IMRT plan, hybrid IMRT plan and VMAT with two 358°arcs. In hybrid IMRT plan, two fields(90°and 270°) were planned to the whole brain. This was used as a base dose plan. Then 5 fields IMRT plan was optimized based on the two fields plan. The dose distribution in the target, the dose to the organs at risk and total MU in three techniques were compared. Results: For the target dose, conformity and homogeneity in PTV, no statistically differences were observed in the three techniques. For the maximum dose in bilateral lens and the mean dose in bilateral eyes, IMRT and h-IMRT plans showed the highest and lowest value respectively. No statistically significant differences were observed in the dose of optic nerve and brainstem. For the monitor units, IMRT and VMAT plans showed the highest and lowest value respectively. Conclusion: For WBRT with simultaneous integrated boost in patients with multiple brain metastases, hybrid IMRT could reduce the doses to lens and eyes. It is feasible for patients with brain metastases.

Extended field intensity-modulated radiotherapy plus concurrent nedaplatin treatment in cervical cancer.

The present study assessed the efficacy and toxicity of definitive extended-field intensity-modulated radiotherapy (EF-IMRT) plus concurrent chemotherapy in cervical cancer. A total of 48 patients with cervical cancer received the planning target volume between 39.6 and 50.4 Gy in 1.8-2.0 Gy daily fractions, while the enlarged pelvic and/or para-aortic nodes were treated with a total dose of 55-60 Gy in 2.0-2.4 Gy daily fractions using simultaneous integrated boost-IMRT. All patients underwent high dose-rate brachytherapy. Concurrent to EF-IMRT, nedaplatin was administered weekly at a median dose of 30 mg/m2 (range, 25-40 mg/m2) for 5 weeks with a total of 150 mg/m2. Of the 48 patients, 46 patients exhibited initial complete responses and 2 patients had partial responses, with a response rate of 100%. After 4-24 months of treatment, 12 patients (27.08%) had local and/or distant failure and 39 patients (81.25%) were alive at the last follow-up. The 12-month overall survival (OS) and disease-free survival (DFS) were 87.5 and 75.8%, respectively, while the 24-month OS and DFS were 69.7 and 49.7%, respectively. Grade ≥3 acute neutropenia and thrombcytopenia occurred in 20 (41.7%) and 4 (8.3%) patients, respectively, while 2 patients (4.2%) developed grade ≥3 diarrhea and 2 (4.2%) had grade ≥3 late toxicities. However, no patients exhibited grade ≥3 vomiting. Thus, concurrent nedaplatin chemotherapy with definitive EF-IMRT was effective and relatively safe for treating patients with cervical cancer. Furthermore, EF-IMRT was able to deliver ≤60 Gy to enlarged para-aortic and/or pelvic nodes using simultaneous integrated boost without increased acute and late gastrointestinal toxicity.

Impact of patient positioning on radiotherapy dose distribution: An assessment in parotid tumor

Purpose: We intended to study the impact of patient positioning on the dose distribution within target volume and organs at risk in patients with parotid malignancies treated with 3D conformal radiotherapy (3D-CRT) with photon wedge pair (WP) or intensity modulated radiotherapy (IMRT). Methods: Three patients with a non-Hodgkin’s lymphoma of the right parotid gland were consecutively immobilized using thermoplastic cast in 2 positions: supine with head in neutral position (HN) and with head turned 90° to the left side (HT). Images for treatment planning purpose were acquired in both positions. For both positions, photon WP plans and 5 field IMRT plans were generated, after contouring clinical target volume (CTV), planning target volume (PTV= CTV + 5 mm margin) and organs at risk (OAR). All plans were evaluated for target coverage and dose to OARs. Results: Both CTV and PTV were apparently larger in HN compared with HT (31.76±8.89 cc, 30.31±7.83 cc and 62.49±19.01 cc, 58.89±15.33 cc) respectively. The CI value for PTV was slightly better for HT compared to HN position in both the WP and IMRT plans. The homogeneity was comparable in both the head positions in case of WP plan. The mean HI of PTV was increased in case of IMRT plan at HT versus HN position (1.108 vs. 1.097). A change in head position from HN to HT with wedge pair plan resulted in a reduction of brainstem D max and D mean . Lesser dose was observed in HN position for contralateral parotid. A difference of 0.9 Gy in the average D max to spinal cord was seen. The values of D mean to mandible, oral cavity, ipsilateral and contralateral cochlea were higher in the HT position. A change in head position from HN to HT with IMRT plan resulted in a dose reduction in average D max to brainstem. The spinal cord D max increased at the HT position by 1.2 Gy. The dose to contralateral parotid and cochlea was comparable in both the positions. However, the D mean to oral cavity was reduced at HT position. Whereas for IMRT versus wedge pair plan at head neutral position average D mean to the contralateral parotid was reduced with the IMRT plan. A considerable reduction in D max to spinal cord and D mean to ipsilateral cochlea was observed. A slight increase in average D max to brainstem and was observed with the IMRT plan. The doses to the remaining OARs were lesser in case of IMRT plan. For IMRT versus wedge pair plan at head tilt position slight increase in average D max to brainstem was observed in case of IMRT plan. A considerable reduction in D max to spinal cord and D mean to ipsilateral cochlea was observed. The doses to the remaining OARs were reduced with IMRT plan. Conclusion: Change in head position from neutral to 90° contralateral tilt for wedge pair plan in parotid tumor may considerably reduce dose to the brainstem and spinal cord with a modest increase in dose to mandible, oral cavity, contralateral parotid, and bilateral cochlea. The alteration in head position has minimal impact on IMRT planning.

Clinical Experience and Evaluation of Patient Treatment Verification With a Transit Dosimeter

To prospectively evaluate a protocol for transit dosimetry on a patient population undergoing intensity modulated radiation therapy (IMRT) and to assess the issues in clinical implementation of electronic portal imaging devices (EPIDs) for treatment verification. Fifty-eight patients were enrolled in the study. Amorphous silicon EPIDs were calibrated for dose and used to acquire images of delivered fields. Measured EPID dose maps were back-projected using the planning computed tomographic (CT) images to calculate dose at prespecified points within the patient and compared with treatment planning system dose offline using point dose difference and point γ analysis. The deviation of the results was used to inform future action levels. Two hundred twenty-five transit images were analyzed, composed of breast, prostate, and head and neck IMRT fields. Patient measurements demonstrated the potential of the dose verification protocol to model dose well under complex conditions: 83.8% of all delivered beams achieved the initial set tolerance level of ΔD of 0±5cGy or %ΔD of 0%±5%. Importantly, the protocol was also sensitive to anatomic changes and spotted that 3 patients from 20 measured prostate patients had undergone anatomic change in comparison with the planning CT. Patient data suggested an EPID-reconstructed versus treatment planning system dose difference action level of 0%±7% for breast fields. Asymmetric action levels were more appropriate for inversed IMRT fields, using absolute dose difference (-2±5cGy) or summed field percentage dose difference (-6%±7%). The invivo dose verification method was easy to use and simple to implement, and it could detect patient anatomic changes that impacted dose delivery. The system required no extra dose to the patient or treatment time delay and so could be used throughout the course of treatment to identify and limit systematic and random errors in dose delivery for patient groups.

A retrospective study on intensity-modulated radiation therapy combined with chemotherapy after D2 radical surgery for gastric carcinoma.

In order to investigate the clinical value of different chemotherapies, the efficacy of intensity-modulated radiation therapy with concurrent chemotherapy following D2 radical surgery for gastric carcinoma was evaluated in this study. A total of 102 patients who underwent D2 radical surgery for gastric carcinoma followed by concurrent chemoradiotherapy (CRT) between January, 2008 and March, 2012, were selected. The 5/7 field intensity-modulated radiation therapy was used, with a planning target volume dose of 45 Gy in 25 fractions over 5 weeks. Among these patients, 45 were administered 400 mg/m2/day fluorouracil and 20 mg/m2/day tetrahydrofurfuryl alcohol through intravenous infusion 4 days before and 3 days after the radiotherapy (F-CRT group), while 57 patients received 825 mg/m2 capecitabine orally twice a day (C-CRT group). The 3-year overall and the disease-free survival rates were 75.5 and 70.5%, respectively. The overall 3-year survival rates of the F-CRT and C-CRT groups were 72.2 and 78.5% (P>0.05), respectively, and the 3-year disease-free survival rates were 67.7 and 72.8% (P>0.05), respectively. No significant differences were observed between the two groups. However, during the concurrent CRT, significant differences were found in the incidence of grade 1-2 haematological toxicity between the F-CRT and C-CRT groups (73.3 vs. 50.9%, respectively; χ2 =5.320, P=0.021). Significant differences were also found in the incidence of grade 1-2 gastrointestinal reactions between the two groups (77.8 vs. 57.9%, respectively; χ2=4.474, P=0.034). Therefore, intensity-modulated radiation therapy combined with concurrent chemotherapy following D2 radical surgery for gastric cancer was found to be safe and effective. In addition, radiotherapy was better tolerated and more likely to be completed using C-CRT rather than F-CRT.

A prospective comparison of acute intestinal toxicity following whole pelvic versus small field intensity-modulated radiotherapy for prostate cancer.

PurposeTo compare the acute intestinal toxicity of whole pelvic (WP) and small field (SF) intensity-modulated radiotherapy (IMRT) for prostate cancer using dosimetric and metabolic parameters as well as clinical findings.MethodsPatients who received IMRT in either a definitive or postoperative setting were prospectively enrolled. Target volume and organs at risk including intestinal cavity (IC) were delineated in every patient by a single physician. The IC volume that received a 10–50 Gy dose at 5-Gy intervals (V10–V50) and the percentage of irradiated volume as a fraction of total IC volume were calculated. Plasma citrulline levels, as an objective biological marker, were checked at three time points: baseline and after exposure to 30 Gy and 60 Gy.ResultsOf the 41 patients, only six experienced grade 1 acute intestinal toxicity. Although all dose–volume parameters were significantly worse following WP than SF IMRT, there was no statistically significant relationship between these dosimetric parameters and clinical symptoms. Plasma citrulline levels did not show a serial decrease by radiotherapy volume difference (WP versus SF) and were not relevant to the irradiated doses.ConclusionGiven that WP had comparable acute intestinal toxicities to those associated with SF, WP IMRT appears to be a feasible approach for the treatment of prostate cancer despite dosimetric disadvantages.

Dosimetric analysis of testicular doses in prostate intensity-modulated and volumetric-modulated arc radiation therapy at different energy levels

The aim of this study is to evaluate the incidental testicular doses during prostate radiation therapy with intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc radiotherapy (VMAT) at different energies. Dosimetric data of 15 patients with intermediate-risk prostate cancer who were treated with radiotherapy were analyzed. The prescribed dose was 78Gy in 39 fractions. Dosimetric analysis compared testicular doses generated by 7-field intensity-modulated radiotherapy and volumetric-modulated arc radiotherapy with a single arc at 6, 10, and 15MV energy levels. Testicular doses calculated from the treatment planning system and doses measured from the detectors were analyzed. Mean testicular doses from the intensity-modulated radiotherapy and volumetric-modulated arc radiotherapy per fraction calculated in the treatment planning system were 16.3 ± 10.3cGy vs 21.5 ± 11.2cGy (p = 0.03) at 6MV, 13.4 ± 10.4cGy vs 17.8 ± 10.7cGy (p = 0.04) at 10MV, and 10.6 ± 8.5cGy vs 14.5 ± 8.6cGy (p = 0.03) at 15MV, respectively. Mean scattered testicular doses in the phantom measurements were 99.5 ± 17.2cGy, 118.7 ± 16.4cGy, and 193.9 ± 14.5cGy at 6, 10, and 15MV, respectively, in the intensity-modulated radiotherapy plans. In the volumetric-modulated arc radiotherapy plans, corresponding testicular doses per course were 90.4 ± 16.3cGy, 103.6 ± 16.4cGy, and 139.3 ± 14.6cGy at 6, 10, and 15MV, respectively. In conclusions, this study was the first to measure the incidental testicular doses by intensity-modulated radiotherapy and volumetric-modulated arc radiotherapy plans at different energy levels during prostate-only irradiation. Higher photon energy and volumetric-modulated arc radiotherapy plans resulted in higher incidental testicular doses compared with lower photon energy and intensity-modulated radiotherapy plans.

Under conditions of large geometric miss, tumor control probability can be higher for static gantry intensity-modulated radiation therapy compared to volume-modulated arc therapy for prostate cancer

The purpose of this work was to compare static gantry intensity-modulated radiation therapy (IMRT) with volume-modulated arc therapy (VMAT) in terms of tumor control probability (TCP) under scenarios involving large geometric misses, i.e., those beyond what are accounted for when margin expansion is determined. Using a planning approach typical for these treatments, a linear-quadratic–based model for TCP was used to compare mean TCP values for a population of patients who experiences a geometric miss (i.e., systematic and random shifts of the clinical target volume within the planning target dose distribution). A Monte Carlo approach was used to account for the different biological sensitivities of a population of patients. Interestingly, for errors consisting of coplanar systematic target volume offsets and three-dimensional random offsets, static gantry IMRT appears to offer an advantage over VMAT in that larger shift errors are tolerated for the same mean TCP. For example, under the conditions simulated, erroneous systematic shifts of 15mm directly between or directly into static gantry IMRT fields result in mean TCP values between 96% and 98%, whereas the same errors on VMAT plans result in mean TCP values between 45% and 74%. Random geometric shifts of the target volume were characterized using normal distributions in each Cartesian dimension. When the standard deviations were doubled from those values assumed in the derivation of the treatment margins, our model showed a 7% drop in mean TCP for the static gantry IMRT plans but a 20% drop in TCP for the VMAT plans. Although adding a margin for error to a clinical target volume is perhaps the best approach to account for expected geometric misses, this work suggests that static gantry IMRT may offer a treatment that is more tolerant to geometric miss errors than VMAT.

A comparative dosimetric study of volumetric-modulated arc therapy vs. fixed field intensity-modulated radiotherapy in postoperative irradiation of stage IB-IIA high-risk cervical cancer.

The aim of the present study was to compare the dosimetry features of volumetric-modulated arc therapy (VMAT) and fixed field intensity-modulated radiotherapy (f-IMRT) in postoperative irradiation of stage IB-IIA high-risk cervical cancer. Fifteen patients exhibiting stage IB-IIA high-risk cervical cancer, who had been treated with postoperative adjuvant concurrent radiochemotherapy, were selected. The clinical target volume (CTV) and organs at risk (OARs) were delineated according to contrast computed tomography images. The planning target volume (PTV) was subsequently produced by using 1 cm uniform expansion of the CTV. The treatment plans were intended to deliver 50 Gy in 25 fractions. The OARs that were contoured included the bladder, rectum, small bowel and femoral heads. Dose volume histograms were used to evaluate the dose distribution in the PTV and OARs. VMAT and f-IMRT treatment plans resulted in similar dose coverage of the PTV. VMAT was superior to f-IMRT in conformity (P<0.05), and resulted in a reduction of OARs irradiated at high dose levels (V40 and V50) compared with f-IMRT (P<0.05), particularly for the bladder. However, the doses of low levels (V10 and V20) delivered to OARs with f-IMRT were slightly reduced compared with VMAT (P<0.05). For ambilateral femoral heads, VMAT demonstrated improved sparing compared with f-IMRT, with regard to D5 (P<0.05). Furthermore, VMAT treatment plans revealed a significant reduction in monitor units (MU) and treatment time. VMAT techniques exhibited similar PTV coverage compared with f-IMRT. At doses of high levels delivered to OARs, VMAT demonstrated improved sparing compared with f-IMRT, particularly for the bladder, while significantly reducing treatment time and MU number.

Detection of IMRT delivery errors based on a simple constancy check of transit dose by using an EPID

Beam delivery errors during intensity modulated radiotherapy (IMRT) were detected based on a simple constancy check of the transit dose by using an electronic portal imaging device (EPID). Twenty-one IMRT plans were selected from various treatment sites, and the transit doses during treatment were measured by using an EPID. Transit doses were measured 11 times for each course of treatment, and the constancy check was based on gamma index (3%/3 mm) comparisons between a reference dose map (the first measured transit dose) and test dose maps (the following ten measured dose maps). In a simulation using an anthropomorphic phantom, the average passing rate of the tested transit dose was 100% for three representative treatment sites (head & neck, chest, and pelvis), indicating that IMRT was highly constant for normal beam delivery. The average passing rate of the transit dose for 1224 IMRT fields from 21 actual patients was 97.6% ± 2.5%, with the lower rate possibly being due to inaccuracies of patient positioning or anatomic changes. An EPIDbased simple constancy check may provide information about IMRT beam delivery errors during treatment.

Comparison of hybrid volumetric modulated arc therapy (VMAT) technique and double arc VMAT technique in the treatment of prostate cancer.

BackgroundVolumetric modulated arc therapy (VMAT) has quickly become accepted as standard of care for the treatment of prostate cancer based on studies showing it is able to provide faster delivery with adequate target coverage and reduced monitor units while maintaining organ at risk (OAR) sparing. This study aims to demonstrate the potential to increase dose conformality with increased planner control and OAR sparing using a hybrid treatment technique compared to VMAT.MethodsEleven patients having been previously treated for prostate cancer with VMAT techniques were replanned with a hybrid technique on Varian Treatment Planning System. Multiple static IMRT fields (2 to 3) were planned initially based on critical OAR to reduce dose but provide some planning treatment volume (PTV) coverage. This was used as a base dose plan to provide 30–35% coverage for a single arc VMAT plan.ResultsThe clinical VMAT plan was used as a control for the purposes of comparison. Average of all OAR sparing between the hybrid technique and VMAT showed the hybrid plan delivering less dose in almost all cases except for V80 of the bladder and maximum dose to right femoral head. PTV coverage was superior with the VMAT technique. Monitor unit differences varied, with the hybrid plan able to deliver fewer units 37% of the time, similar results 18% of the time, and higher units 45% of the time. On average, the hybrid plan delivered 10% more monitor units.ConclusionsThe hybrid plan can be delivered in a single gantry rotation combining aspects of VMAT with regions of dynamic intensity modulated radiation therapy (IMRT) within the treatment arc.

A modular approach to intensity-modulated arc therapy optimization with noncoplanar trajectories

Utilizing noncoplanar beam angles in volumetric modulated arc therapy (VMAT) has the potential to combine the benefits of arc therapy, such as short treatment times, with the benefits of noncoplanar intensity modulated radiotherapy (IMRT) plans, such as improved organ sparing. Recently, vendors introduced treatment machines that allow for simultaneous couch and gantry motion during beam delivery to make noncoplanar VMAT treatments possible. Our aim is to provide a reliable optimization method for noncoplanar isocentric arc therapy plan optimization. The proposed solution is modular in the sense that it can incorporate different existing beam angle selection and coplanar arc therapy optimization methods.Treatment planning is performed in three steps. First, a number of promising noncoplanar beam directions are selected using an iterative beam selection heuristic; these beams serve as anchor points of the arc therapy trajectory. In the second step, continuous gantry/couch angle trajectories are optimized using a simple combinatorial optimization model to define a beam trajectory that efficiently visits each of the anchor points. Treatment time is controlled by limiting the time the beam needs to trace the prescribed trajectory. In the third and final step, an optimal arc therapy plan is found along the prescribed beam trajectory. In principle any existing arc therapy optimization method could be incorporated into this step; for this work we use a sliding window VMAT algorithm.The approach is demonstrated using two particularly challenging cases. The first one is a lung SBRT patient whose planning goals could not be satisfied with fewer than nine noncoplanar IMRT fields when the patient was treated in the clinic. The second one is a brain tumor patient, where the target volume overlaps with the optic nerves and the chiasm and it is directly adjacent to the brainstem.Both cases illustrate that the large number of angles utilized by isocentric noncoplanar VMAT plans can help improve dose conformity, homogeneity, and organ sparing simultaneously using the same beam trajectory length and delivery time as a coplanar VMAT plan.

SU‐E‐T‐59: A Novel Multi‐Beam Dynamic IMRT with Fixed‐Jaw Technique for Left Breast Cancer Patients with Regional Lymph Nodes Radiotherapy

Purpose:This study was to investigate the dosimetric benefit of a novel intensity modulated radiation therapy (IMRT) technique for irradiating the left breast and regional lymph node (RLN).Methods:The breast and RLN (internal mammary node and periclavicular node) and normal tissue were contoured for 16 consecutive left‐sided breast cancer patients previously treated with RT after lumpectomy. Nine equi‐spaced fields IMRT (9 ‐field IMRT), tangential multi‐beam IMRT (tangential‐IMRT) and IMRT with fixed‐jaw technique (FJT‐IMRT) were developed and compared with three‐dimensional conformal RT (3DCRT). Prescribed dose was 50 Gy in 25 fractions. Dose distributions and dose volume histograms were used to evaluate plans.Results:All IMRTs achieved similar target coverage and substantially reduced heart V30 and V20 compared to the 3DCRT. The average heart mean dose had different changes, which were 9.0Gy for 9‐field IMRT, 5.7Gy for tangential‐IMRT and 4.2Gy for FJT‐IMRT. For the contralateral lung and breast, the 9‐field IMRT has the highest mean dose; and the FJT‐IMRT and tangential‐IMRT had similar lower value. For the thyroid, both 9‐field IMRT and FJT‐IMRT had similar V30 (20% and 22%) and were significantly lower than that of 3DCRT (34%) and tangential‐IMRT (46%). Moreover, the thyroid mean dose of FJT‐IMRT is the lowest. For cervical esophagus and humeral head, the FJT‐IMRT also had the best sparing.Conclusion:All 9‐field IMRT, tangential‐IMRT and FJT‐IMRT had superiority for targets coverage and substantially reduced the heart volume of high dose irradiation. The FJT‐IMRT showed advantages of avoiding the contralateral breast and lung irradiation and decreasing the thyroid, humeral head and cervical esophagus radiation dose at the expense of a slight monitor units (MUs) increasing.

SU‐E‐T‐29: A Dosimetric Study of Volumetric Modulated Arc Therapy with Simultaneous Integrated Boost for Rectal Cancer

Purpose:To compare the dosimetric differences among fixed field intensity‐modulated radiotherapy (IMRT) and double‐arc volumetricmodulated arc therapy (VMAT) plans with simultaneous integrated boost in rectal cancer.Methods:Ten patients with rectal cancer previously treated with IMRT were included in this analysis. For each patient, two treatment techniques were designed for each patient: the fixed 7 fields IMRT and double‐arc VMAT with RapidArc technique. The treatment plan was designed to deliver in one process with simultaneous integrated boost (SIB). The prescribed doses to the planning target volume of the subclinical disease (PTV1) and the gross disease (PTV2) were 45 Gy and 55 Gy in 25 fractions, respectively. The dose distribution in the target, the dose to the organs at risk, total MU and the delivery time in two techniques were compared to explore the dosimetric differences.Results:For the target dose and homogeneity in PTV1 and PTV2, no statistically differences were observed in the two plans. VMAT plans showed a better conformity in PTV1. VMAT plans reduced the mean dose to bladder, small bowel, femur heads and iliac wings. For iliac wings, VMAT plans resulted in a statistically significant reduction in irradiated volume of 15 Gy, 20 Gy, 30 Gy but increased the 10 Gy irradiated volume. VMAT plans reduced the small bowel irradiated volume of 20 Gy and 30 Gy. Compared with IMRT plans, VMAT plans showed a significant reduction of monitor units by nearly 30% and reduced treatment time by an average of 70%Conclusion:Compared to IMRT plans, VMAT plans showed the similar target dose and reduced the dose of the organs at risk, especially for small bowel and iliac wings. For rectal cancer, VMAT with simultaneous integrated boost can be carried out with high quality and efficiency.

WE-D-BRA-05: Pseudo In Vivo Patient Dosimetry Using a 3D-Printed Patient-Specific Phantom

Purpose: To test the feasibility of using 3D-printed patient-specific phantoms for intensity-modulated radiation therapy (IMRT) quality assurance (QA). Methods: We created a patient-specific whole-head phantom using a 3D printer. The printer data file was created from high-resolution DICOM computed tomography (CT) images of 3-year old child treated at our institution for medulloblastoma. A custom-modified extruder system was used to create tissue-equivalent materials. For the printing process, the Hounsfield Units from the CT images were converted to proportional volumetric densities. A 5-field IMRT plan was created from the patient CT and delivered to the 3D- phantom. Dose was measured by an ion chamber placed through the eye. The ion chamber was placed at the posterior edge of the planning target volume in a high dose gradient region. CT scans of the patient and 3D-phantom were fused by using commercial treatment planning software (TPS). The patient's plan was calculated on the phantom CT images. The ion chamber's active volume was delineated in the TPS; dose per field and total dose were obtained. Measured and calculated doses were compared. Results: The 3D-phantom dimensions and tissue densities were in good agreement with the patient. However, because of a printing error, there was a large discrepancy in the density in the frontal cortex. The calculated and measured treatment plan doses were 1.74 Gy and 1.72 Gy, respectively. For individual fields, the absolute dose difference between measured and calculated values was on average 3.50%. Conclusion: This study demonstrated the feasibility of using 3D-printed patient-specific phantoms for IMRT QA. Such phantoms would be particularly advantageous for complex IMRT treatment plans featuring high dose gradients and/or for anatomical sites with high variation in tissue densities. Our preliminary findings are promising. We anticipate that, once the printing process is further refined, the agreement between measured and calculated doses will improve.

A dual two dimensional electronic portal imaging device transit dosimetry model based on an empirical quadratic formalism.

This study describes a two dimensional electronic portal imaging device (EPID) transit dosimetry model that can predict either: (1) in-phantom exit dose, or (2) EPID transit dose, for treatment verification. The model was based on a quadratic equation that relates the reduction in intensity to the equivalent path length (EPL) of the attenuator. In this study, two sets of quadratic equation coefficients were derived from calibration dose planes measured with EPID and ionization chamber in water under reference conditions. With two sets of coefficients, EPL can be calculated from either EPID or treatment planning system (TPS) dose planes. Consequently, either the in-phantom exit dose or the EPID transit dose can be predicted from the EPL. The model was tested with two open, five wedge and seven sliding window prostate and head and neck intensity-modulated radiation therapy (IMRT) fields on phantoms. Results were analysed using absolute gamma analysis (3%/3 mm). The open fields gamma pass rates were >96.8% for all comparisons. For wedge and IMRT fields, comparisons between predicted and TPS-computed in-phantom exit dose resulted in mean gamma pass rate of 97.4% (range, 92.3-100%). As for the comparisons between predicted and measured EPID transit dose, the mean gamma pass rate was 97.5% (range, 92.6-100%). An EPID transit dosimetry model that can predict in-phantom exit dose and EPID transit dose was described and proven to be valid. The described model is practical, generic and flexible to encourage widespread implementation of EPID dosimetry for the improvement of patients' safety in radiotherapy.

Feasibility of using Geant4 Monte Carlo simulation for IMRT dose calculations for the Novalis Tx with a HD-120 multi-leaf collimator

The aim of this study was to develop an independent dose verification system by using a Monte Carlo (MC) calculation method for intensity modulated radiation therapy (IMRT) conducted by using a Varian Novalis Tx (Varian Medical Systems, Palo Alto, CA, USA) equipped with a highdefinition multi-leaf collimator (HD-120 MLC). The Geant4 framework was used to implement a dose calculation system that accurately predicted the delivered dose. For this purpose, the Novalis Tx Linac head was modeled according to the specifications acquired from the manufacturer. Subsequently, MC simulations were performed by varying the mean energy, energy spread, and electron spot radius to determine optimum values of irradiation with 6-MV X-ray beams by using the Novalis Tx system. Computed percentage depth dose curves (PDDs) and lateral profiles were compared to the measurements obtained by using an ionization chamber (CC13). To validate the IMRT simulation by using the MC model we developed, we calculated a simple IMRT field and compared the result with the EBT3 film measurements in a water-equivalent solid phantom. Clinical cases, such as prostate cancer treatment plans, were then selected, and MC simulations were performed. The accuracy of the simulation was assessed against the EBT3 film measurements by using a gamma-index criterion. The optimal MC model parameters to specify the beam characteristics were a 6.8-MeV mean energy, a 0.5-MeV energy spread, and a 3-mm electron radius. The accuracy of these parameters was determined by comparison of MC simulations with measurements. The PDDs and the lateral profiles of the MC simulation deviated from the measurements by 1% and 2%, respectively, on average. The computed simple MLC fields agreed with the EBT3 measurements with a 95% passing rate with 3%/3-mm gamma-index criterion. Additionally, in applying our model to clinical IMRT plans, we found that the MC calculations and the EBT3 measurements agreed well with a passing rate of greater than 95% on average with a 3%/3-mm gamma-index criterion. In summary, the Novalis Tx Linac head equipped with a HD-120 MLC was successfully modeled by using a Geant4 platform, and the accuracy of the Geant4 platform was successfully validated by comparisons with measurements. The MC model we have developed can be a useful tool for pretreatment quality assurance of IMRT plans and for commissioning of radiotherapy treatment planning.

Intensity modulated radiotherapy versus volumetric modulated arc therapy in breast cancer: A comparative dosimetric analysis

Purpose: Intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) has the capacity to optimize the dose distribution. We analyzed the dosimetric differences of plans in treatment planning system (TPS) between VMAT and IMRT in treating breast cancer. Methods : Fourteen patients were simulated, planned, and treated with VMAT using single, double or partial arcs. IMRT treatments were generated using 4 to 5 tangential IMRT fields for the same patients. All treatment plans were planned for 50 Gy in 25 fractions. The VMAT and IMRT plans were compared using the planning target volume (PTV) dose and doses to the other organs at risk (OARs). Results : For the PTV, comparable minimum, mean, maximum, median, and modal dose as well equivalent sphere diameter of the structure (Equis) were observed between VMAT and IMRT plans and found that these values were significantly equal in both techniques. The right lung mean and modal doses were considerably higher in VMAT plans while maximum value was considerably lower when compared with IMRT plans. The left lung mean and modal doses were higher with VMAT while maximum doses were higher in IMRT plans. The mean dose to the heart and maximum dose to the spinal cord was lower with IMRT. The mean dose to the body was higher in VMAT plans while the maximum dose was higher in IMRT plans. Conclusion : Four field tangential IMRT delivered comparable PTV dose with generally less dose to normal tissues in our breast cancer treatment study. The IMRT plans typically had more favourable dose characteristics to the lung, heart, and spinal cord and body dose when compared with VMAT. The only minor advantage of VMAT for breast cases was slightly better PTV coverage.

First dose-map measured with a polycrystalline diamond 2D dosimeter under an intensity modulated radiotherapy beam

A prototype of bidimensional dosimeter made on a 2.5×2.5cm2 active area polycrystalline Chemical Vapour Deposited (pCVD) diamond film, equipped with a matrix of 12×12 contacts connected to the read-out electronics, has been used to evaluate a map of dose under Intensity Modulated Radiation Therapy (IMRT) fields for a possible application in pre-treatment verifications of cancer treatments. Tests have been performed under a 6–10 MVRX beams with IMRT fields for prostate and breast cancer. Measurements have been taken by measuring the 144 pixels in different positions, obtained by shifting the device along the x/y axes to span a total map of 14.4×10cm2. Results show that absorbed doses measured by our pCVD diamond device are consistent with those calculated by the Treatment Planning System (TPS).